This invention generally relates to light emitter diode (LED) backlighting of liquid crystal displays (LCD), including the driving and control of those LED's.
LCD displays comprise a significant percentage of the market for display sizes large and small, including displays for cash registers, product dispensers, gas pumps, computer displays (laptop and stand alone), and flat panel televisions. The lighting for these LCD displays is typically provided by a backlight unit installed underneath a display panel, wherein the backlight unit includes one or more light sources and a light diffusion means for providing a uniformly distributed light source. Depending on the position of the light source(s), backlight units can be categorized as either edge type or direct type. Edge type backlight units typically consist of one or more edge rails where the light sources are located.
A significant percentage of backlight units for LCD displays use cold cathode fluorescent lamps (CCFL) for their light sources. CCFL light sources can require a starting voltage of approximately 1,200V and a sustaining voltage of approximately 500V. Recently, LED's, including High Bright LED's (HBLEDs), have been used as light sources in backlight units. Generally, LED backlight units provide advantages over CCFL backlight units based on the LED's inherent compactness, solid state nature, and operation at lower voltage levels and temperatures without the need for ignition voltage or a warm-up period. Given these characteristics, it would be advantageous to provide an LED backlight system to be used in lieu of or as a replacement/retrofit for CCFL backlight units or similar backlight units without having to necessarily replace or modify all of the hardware and optics typically used for the LCD display, including, for example, the front frame, lens sheet, diffuser sheet, light pipe, reflector sheet, reflector, and back frame.
Although there are certain advantages, the use of LED backlight units also present challenges with respect to thermal management, diffusion of point source light, driving, and control to provide the required performance. LED's generally provide a predominantly fixed voltage drop over a specified range of drive current levels. Accordingly, to drive LED backlight units, it is often necessary to provide a constant current source to provide the desired brightness. Given these electrical properties, each series connection of LED's in an LED backlight unit requires constant current and a forward voltage drop that increases as the number of LED's in the particular series increases. For example, for a series connection of 24 HBLED's, where each HBLED is driven at 1.0 amps and has a fixed voltage drop of 3 volts, the driving system must be capable of providing 72 volts for that particular series of HBLED's. Since many applications only have access to input voltages in the range of +5V to +24V with +12V being the most common, many LED backlight applications will require a driver unit that can develop the required forward voltage.
In order to minimize the required forward voltage for a series connection of a significant number of LED's required to provide edge lighting (e.g., 24 HBLED's requiring a forward voltage of approximately 72 volts), prior art solutions break the entire series connection of LED's into parallel banks of series LED's (e.g., 4 banks of 6 LED's), with each bank requiring a lower forward voltage (e.g., 18 volts). Typically, any LED backlight solution requiring +42V or greater has resulted in the use of parallel banks of series LED's.
One apparent advantage of the use of multiple banks of LED's is minimizing the impact of a failure of a series of LED's. In order to maintain the proper current balance in this arrangement for each parallel bank, however, it is necessary to provide a series dropping resistor between each of the banks, which leads to inefficient power delivery based on the losses across these resistors. This banked configuration also requires additional wiring for each separate bank. In order to improve power efficiency and thermal management, and minimize wiring requirements, it would be advantageous to provide an LED backlight system using a series connection that avoided the use of resistors but still provided protection against losing an entire series of LED's in the event of a failure as well as the appropriate driver unit for this application.
Another consideration in using LED backlight modules is accomplishing the required intensity or dimming control for a particular application. Intensity or dimming control is necessary for many backlight applications. In many battery-operated applications, dimming provides a benefit by extending battery life since the backlight unit consumes a significant portion of the system's power budget. While some of these applications only require fairly modest dimming ranges on order of 5 to 1 or less, other applications, including those that need to be viewed during the day or night may require a dimming range as wide as 1,000 to 1. As with CCFL's, dimming of LED's can be accomplished by varying the constant current level (amplitude) driving the LED's. However, operating the LED or series connection of LED's at relatively low current (e.g., 1 to 10% of full output brightness) can result in luminous instability, which in turn results in flickering observed by the human eye. Accordingly, while varying the amplitude of the constant current will be effective to accomplish dimming over the majority of the desired range, dimming at the lowest levels will be compromised. In order to improve dimming control and performance, it would be advantageous to provide a driver unit that would allow for satisfactory dimming control across the entire desired dimming range, including at the lowest levels.